Hi Siobhan, not sure if you remember me but my DS is a little bit older than your DS and we also had many issues with MSPI, reflux, etc. Congratulations on your pregnancy!
We were scared too before having DD2 but turns out she is an angel of a baby. I do not have any experience with lactose intolerance vs BF baby so I cannot comment on that, but with regards to MSPI, when DD2 was 2 weeks old, we found she was spitting up a lot and although she did not seem in pain, I decided to be proactive and remove all dairy and beef from my diet. She immediately stopped spitting up. I remained dairy and beef free for about 6 months and then slowly reintroduced beef, cheese and yogurt first, and just recently milk and cream and she's been doing fine.
I truly believe that acting early is really important, because it prevents their litle digestive system from being too irritated and therefore long to heal.
best of luck to you and enjoy the newborn snuggles!